Redheaded-ness is a recessive trait. Obviously a society could just kill every baby that comes out with a different hair color, but is there any passive way for a human ethnic group to be 100% ginger?
1-Isolation, which means incest which means ''keeping the bloodline pure''
real world example : take mongolians and trap them into tiny island with a xenophobic religion and culture and the result will be the japanese race.
2-sexual selection, which means really horny people with really high standards and integrity
real world examples : the meme of girls not dating anyone shorter than 6 feet is a modern example of sexual selection in the human species.
3-religion/culture , which is basically sexual selection without the horny part
real world examples : dating people of different races is considered a fetish and frowned uppon by most cultures.
for better results mix them all up and voila' you've got your redhead ethnicity.
Neanderthals where pretty isolated and low in number, most, almost all of them where red headed, so much so that for decades scientists believed modern gingers got their red hair from neanderthal hybrid DNA.
also if we were to ignore the genetics tag, then just have a culture that bathes in either ammonia or peroxide hydrogen or maybe simply ashes and coal if they are really primitive. Oxydized hair turns red or orange. (WARNING those substances can and will bleach skin a few tones)